The Universal Serial Bus (USB) standard was an improvement over the conventional serial bus standard, partly due to the fact that the USB cable could supply power to the peripheral device. As a result, peripheral manufacturers could supply new peripherals which did not need there own respective power supply adapter.
The USB standard imposes a limit on the maximum allowable current that a peripheral device can draw from the VBUS. According to the USB standard, a device attached to the VBUS must limit its current draw to 100 mA for low power bus-powered devices, and 500 mA for high power bus-powered devices. This limitation prevents peripheral devices which have excessive load current demands from damaging the USB connection.
A peripheral device might draw an excessive load current (“in-rush current”) only for the interim period required to charge the internal capacitors of the peripheral device. Thereafter, the current drawn by the peripheral device might be within the maximum allowable limit. However, the interim period may be greater than that tolerated by the USB standard. As a result, the number of peripheral devices that can take advantage of the power supply capabilities of the USB standard is limited.
Therefore, there is a need for a mechanism for powering a load from a current-limited power supply where the interim load current demands of the load might exceed the current source capabilities of the power supply. Moreover, there is a need for an improved mechanism for powering a peripheral device having a large in-rush current draw from a USB connection.